Global Biogeochemical Cycles最新文献

Wetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra-high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUP_{High O/C} compounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine-to-coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling.

Lithogenic materials such as terrigenous lithogenic particles (TLP) can efficiently promote the formation and sinking of mineral-associated marine organic matter, acting as important ballast and potentially playing an important role in the global carbon cycle. To assess the influence of TLP on fluxes of particulate organic carbon (POC) and other biogeochemical cycles, we construct TLP forcing fields based on global riverine suspended sediment data and then apply them to the Community Earth System Model, version 2 (CESM2) modified with the TLP ballasting effect term. Simulations forced by different concentrations of TLP transported in the surface ocean or along the bottom of continental shelves and slopes are conducted. When the TLP transports seaward along the bottom, simulated POC fluxes at 100 and 2,000 m decrease about 11% and 19%, respectively, for the global ocean, and about 9% and 12%, respectively, for the oceanic regions of continental margins. The initial abiotic ballast processes triggered by TLP input increase POC fluxes, causing additional removal and burial of dissolved iron in continental margins. This further enhances the accumulation of macronutrients in the upwelling regions and their advection transport to neighboring subtropical gyres, thus altering regional productivity when simulations reach quasi-equilibrium. When consider the impacts of TLP in simulations, the simulated POC flux exhibits an increase in subtropical gyres but a decrease in tropical Pacific and mid-high latitude regions. The present work highlights the importance of TLP in global biogeochemical cycles, suggesting that the amount of carbon sequestration might be overestimated without TLP in models.

Radiocarbon (¹⁴C) in corals can be used as a relatively high-sensitivity indicator of vertical and horizontal advection of water masses, which contributes to the understanding of ocean circulation. In this study, we reconstruct Kuroshio and Ryukyu current transport with a seasonal resolution Δ¹⁴C record spanning 1947–2009. This record covers the beginning of the atomic era and was obtained from a coral on Kikai Island in the south of Japan. The Kikai Δ¹⁴C curve features a newly discovered Δ¹⁴C spike in July 1955, a rapid increase after 1962, and a steady decrease after 1980. The spike in 1955 may directly reflect ocean current transport. The lack of periodicity in the Δ¹⁴C record suggests the existence of mesoscale eddies and the complexity of Kuroshio and Ryukyu current transport. In addition, comparing the high-resolution Δ¹⁴C of Kikai and Ishigaki islands, both situated along the path of the Kuroshio, reveals the influence of Pacific Decadal Oscillation and El Niño-Southern Oscillation on the Kuroshio and Ryukyu currents. This suggests that seasonally resolved Δ¹⁴C in corals along an ocean current can produce a long-term record of ocean mixing that responds to climate variability.

As part of the second phase of the Regional Carbon Cycle Assessment and Processes project (RECCAP2), we present an assessment of the carbon cycle of the Atlantic Ocean, including the Mediterranean Sea, between 1985 and 2018 using global ocean biogeochemical models (GOBMs) and estimates based on surface ocean carbon dioxide (CO₂) partial pressure (pCO₂ products) and ocean interior dissolved inorganic carbon observations. Estimates of the basin-wide long-term mean net annual CO₂ uptake based on GOBMs and pCO₂ products are in reasonable agreement (−0.47 ± 0.15 PgC yr⁻¹ and −0.36 ± 0.06 PgC yr⁻¹, respectively), with the higher uptake in the GOBM-based estimates likely being a consequence of a deficit in the representation of natural outgassing of land derived carbon. In the GOBMs, the CO₂ uptake increases with time at rates close to what one would expect from the atmospheric CO₂ increase, but pCO₂ products estimate a rate twice as fast. The largest disagreement in the CO₂ flux between GOBMs and pCO₂ products is found north of 50°N, coinciding with the largest disagreement in the seasonal cycle and interannual variability. The mean accumulation rate of anthropogenic CO₂ (C_ant) over 1994–2007 in the Atlantic Ocean is 0.52 ± 0.11 PgC yr⁻¹ according to the GOBMs, 28% ± 20% lower than that derived from observations. Around 70% of this C_ant is taken up from the atmosphere, while the remainder is imported from the Southern Ocean through lateral transport.

Peatlands store organic carbon available for decomposition and transfer to neighboring water bodies, which can ultimately generate carbon dioxide (CO₂) and methane (CH₄) emissions. The objective of this study was to clarify the biogeochemical functioning of open-water peatland pools and their influence on carbon budgets at the ecosystem and global scale. Continuously operated automated equipment and monthly manual measurements were used to describe the CO₂ and CH₄ dynamics in boreal ombrotrophic peatland pools and porewater (Québec, Canada) over the growing seasons 2019 and 2020. The peat porewater stable carbon isotope ratios (δ¹³C) for both CO₂ (median δ¹³C-CO₂: −3.8‰) and CH₄ (median δ¹³C-CH₄: −64.30‰) suggested that hydrogenotrophic methanogenesis was the predominant degradation pathway in peat. Open-water pools were supersaturated in CO₂ and CH₄ and received most of these dissolved carbon greenhouse gases (C-GHG) from peat porewater input. Throughout the growing season, higher CO₂ concentrations and fluxes in pools were measured when the water table was low—suggesting a steady release of CO₂ from deep peat porewater. Higher CH₄ ebullition and diffusion occurred in August when bottom water and peat temperatures were the highest. While this study demonstrates that peatland pools are chimneys of CO₂ and CH₄ stored in peat, it also shows that the C-GHG concentrations and flux rates in peat pools are comparable to other aquatic systems of the same size. Although peatlands are often considered uniform entities, our study highlights their biogeochemical heterogeneity, which, if considered, substantially influences their net carbon balance with the atmosphere.

As major sites of carbon burial and remineralization, continental margins are key components of the global carbon cycle. However, heterogeneous sources of organic matter (OM) and depositional environments lead to complex spatial patterns in sedimentary organic carbon (OC) content and composition. To better constrain the processes that control OM cycling, we focus on the East Asian marginal seas as a model system, where we compiled extensive data on the OC content, bulk isotopic composition (δ¹³C and Δ¹⁴C), total nitrogen, and mineral surface area of surficial sediments from previous studies and new measurements. We developed a spatial machine learning modeling framework to predict the spatial distribution of these parameters and identify regions where sediments with similar geochemical signatures drape the seafloor (i.e., “isodrapes”). We demonstrate that both provenance (44%–77%) and hydrodynamic processes (22%–53%) govern the fate of OM in this margin. Hydrodynamic processes can either promote the degradation of OM in mobile mud-belts or preserve it in stable mud-deposits. The distinct isotopic composition of OC sources from marine productivity and individual rivers regulates the age and reactivity of OM deposited on the sea-floor. The East Asian marginal seas can be separated into three main isodrapes: hydrodynamically energetic shelves with coarser-grained sediment depleted in OC, OM-enriched mud deposits, and a deep basin with fine-grained sediments and aged OC affected by long oxygen exposure times and petrogenic input from rivers. This study confirms that both hydrodynamic processes and provenance should be accounted for to understand the fate of OC in continental margins.

As part of the REgional Carbon Cycle Assessment and Processes Phase 2 (RECCAP2) project, we developed a comprehensive African Greenhouse gases (GHG) budget covering 2000 to 2019 (RECCAP1 and RECCAP2 time periods), and assessed uncertainties and trends over time. We compared bottom-up process-based models, data-driven remotely sensed products, and national GHG inventories with top-down atmospheric inversions, accounting also for lateral fluxes. We incorporated emission estimates derived from novel methodologies for termites, herbivores, and fire, which are particularly important in Africa. We further constrained global woody biomass change products with high-quality regional observations. During the RECCAP2 period, Africa's carbon sink capacity is decreasing, with net ecosystem exchange switching from a small sink of −0.61 ± 0.58 PgC yr⁻¹ in RECCAP1 to a small source in RECCAP2 at 0.16 (−0.52/1.36) PgC yr⁻¹. Net CO₂ emissions estimated from bottom-up approaches were 1.6 (−0.9/5.8) PgCO₂ yr⁻¹, net CH₄ were 77 (56.4/93.9) TgCH₄ yr⁻¹ and net N₂O were 2.9 (1.4/4.9) TgN₂O yr⁻¹. Top-down atmospheric inversions showed similar trends. Land Use Change emissions increased, representing one of the largest contributions at 1.7 (0.8/2.7) PgCO₂eq yr⁻¹ to the African GHG budget and almost similar to emissions from fossil fuels at 1.74 (1.53/1.96) PgCO₂eq yr⁻¹, which also increased from RECCAP1. Additionally, wildfire emissions decreased, while fuelwood burning increased. For most component fluxes, uncertainty is large, highlighting the need for increased efforts to address Africa-specific data gaps. However, for RECCAP2, we improved our overall understanding of many of the important components of the African GHG budget that will assist to inform climate policy and action.

Comprehensive seasonal observation is essential for accurately quantifying methane (CH₄) emissions from ponds and lakes in permafrost regions. Although CH₄ emissions during ice thaw are important and highly variable in high-latitude freshwater ponds and lakes (north of ∼50°N), their contribution is seldom included in estimates of aquatic-atmospheric CH₄ exchange across different alpine ecosystems. Here, we characterized annual CH₄ emissions, including emissions during ice thaw, from ponds and lakes across four alpine vegetation zones in the Qinghai-Tibet Plateau (QTP) permafrost region. We observed significant spatial variability in annual CH₄ emission rates (8.44−421.05 mmol m⁻² yr⁻¹), CH₄ emission rates during ice thaw (0.26−144.39 mmol m⁻² yr⁻¹), and the contribution of CH₄ emissions during ice thaw to annual emissions (3−33%) across different vegetation zones. Dissolved oxygen concentration under ice, along with substrate availability and water salinity, played critical roles in influencing CH₄ flux during ice thaw. We estimated annual CH₄ emissions from ponds and lakes in the QTP permafrost region as 0.04 (0.03−0.05) Tg CH₄ yr⁻¹ (median (first quartile−third quartile)), with approximately 20% occurring during ice thaw. Notably, the average areal CH₄ emission rate from ponds and lakes in the QTP permafrost region amounts to only 8% of that from high-latitude waterbodies, primarily due to the dominance of large saline lakes with lower CH₄ emission rates in the alpine permafrost region. Our findings emphasize the significance of incorporating comprehensive seasonal observation of CH₄ emissions across different vegetation zones in better predicting CH₄ emissions from alpine ponds and lakes.

The northern permafrost region has been projected to shift from a net sink to a net source of carbon under global warming. However, estimates of the contemporary net greenhouse gas (GHG) balance and budgets of the permafrost region remain highly uncertain. Here, we construct the first comprehensive bottom-up budgets of CO₂, CH₄, and N₂O across the terrestrial permafrost region using databases of more than 1000 in situ flux measurements and a land cover-based ecosystem flux upscaling approach for the period 2000–2020. Estimates indicate that the permafrost region emitted a mean annual flux of 12 (−606, 661) Tg CO₂–C yr⁻¹, 38 (22, 53) Tg CH₄–C yr⁻¹, and 0.67 (0.07, 1.3) Tg N₂O–N yr⁻¹ to the atmosphere throughout the period. Thus, the region was a net source of CH₄ and N₂O, while the CO₂ balance was near neutral within its large uncertainties. Undisturbed terrestrial ecosystems had a CO₂ sink of −340 (−836, 156) Tg CO₂–C yr⁻¹. Vertical emissions from fire disturbances and inland waters largely offset the sink in vegetated ecosystems. When including lateral fluxes for a complete GHG budget, the permafrost region was a net source of C and N, releasing 144 (−506, 826) Tg C yr⁻¹ and 3 (2, 5) Tg N yr⁻¹. Large uncertainty ranges in these estimates point to a need for further expansion of monitoring networks, continued data synthesis efforts, and better integration of field observations, remote sensing data, and ecosystem models to constrain the contemporary net GHG budgets of the permafrost region and track their future trajectory.

Photosynthesis in the surface ocean and subsequent export of a fraction of this fixed carbon leads to carbon dioxide sequestration in the deep ocean. Ecological relationships among plankton functional groups and theoretical relationships between particle size and sinking rate suggest that carbon export from the euphotic zone is more efficient when communities are dominated by large organisms. However, this hypothesis has never been tested against measured size spectra spanning the >5 orders of magnitude found in plankton communities. Using data from five ocean regions (California Current Ecosystem, North Pacific subtropical gyre, Costa Rica Dome, Gulf of Mexico, and Southern Ocean subtropical front), we quantified carbon-based plankton size spectra from heterotrophic bacteria to metazoan zooplankton (size class cutoffs varied slightly between regions) and their relationship to net primary production and sinking particle flux. Slopes of the normalized biomass size spectra (NBSS) varied from −1.6 to −1.2 (median slope of −1.4 equates to large 1–10 mm organisms having a biomass equal to only 7.6% of the biomass in small 1–10 μm organisms). Net primary production was positively correlated with the NBSS slope, with a particularly strong relationship in the microbial portion of the size spectra. While organic carbon export co-varied with NBSS slope, we found only weak evidence that export efficiency is related to plankton community size spectra. Multi-variate statistical analysis suggested that properties of the NBSS added no explanatory power over chlorophyll, primary production, and temperature. Rather, the results suggest that both plankton size spectra and carbon export increase with increasing system productivity.